1. Field of the Invention
The present invention relates to a magnetoresistance device, and more particularly, to a magnetoresistance device with an improved MR characteristic and thermal stability, obtained by adjusting a composition of an upper layer, a lower layer, or a tunneling barrier layer of the magnetoresistance device.
2. Description of the Related Art
The operational principle of a magnetoresistance device is based on the way an electrical resistance of a specific material is varied in the presence of a magnetic field. In recent years, with the rapid developments in thin-film deposition techniques and surface processing techniques in ultra-high vacuum, it has become possible to finely grow a magnetic thin film to a thickness of several nanometers(nm), which is the equivalent of an interaction distance between spins, and therefore to manufacture a high-performance magnetoresistance device. Thus, various phenomena that had not been seen in bulk-type magnetic materials were found and began to be applied in household appliances and industrial components. For example, a magnetic recording head and a magnetic random access memory (MRAM) are kinds of high-performance magnetoresistance devices for writing data on an ultra-high-density data storage device.
Other examples of high-performance magnetoresistance devices widely used are a magnetoresistance head, which is used to read/record data stored on a data storage medium such as a hard disk drive (HDD), a giant magnetoresistance head (GMR head), and a tunneling magnetoresistance head (TMR head).
In the GMR head, a resistance is varied according to magnetization arrangements of two magnetic layers, which can be explained using a spin dependent scattering. Also, in the TMR head, a tunneling current is varied according to a relative magnetization direction of a ferromagnetic material in a structure formed of two magnetic layers and an insulator therebetween.
FIGS. 1A and 1B are cross-sectional views of a conventional magnetoresistance device. FIG. 1A illustrates a conventional GMR device. Typically, the GMR device is widely used as a spin valve magnetoresistance device. The spin valve magnetoresistance device has various shapes and thus is not limited to the shape shown in FIG. 1A.
Referring to FIG. 1A, a spin valve magnetoresistance device 10 comprises a lower layer 12, a first ferromagnetic layer 13, a spacer layer 14, a second ferromagnetic layer 15, a semi-ferromagnetic layer 16, and an upper layer 17, which are sequentially stacked on a substrate 11, for example, a Si wafer. The lower layer 12 is typically formed of tantalum (Ta). The first ferromagnetic layer 13 is formed of a ferromagnetic material, such as a CoFe alloy and is referred as a free layer since its magnetization direction is varied by an applied magnetic field. The spacer layer 14 is formed of a nonmagnetic material, such as Cu, and separates the first ferromagnetic layer 13 from the second ferromagnetic layer 15. The second ferromagnetic layer 15 is formed of a ferromagnetic material, such as a CoFe alloy and is referred to as a fixed layer. The semi-ferromagnetic layer 16 is mainly formed of a Mn alloy, such as an IrMn alloy, a FeMn alloy, or a NiMn alloy, and used to fix a magnetization direction of the second ferromagnetic layer 15. The semi-ferromagnetic layer 16 constitutes a sensing portion along with the first ferromagnetic layer 13, the spacer layer 14, and the second ferromagnetic layer 15. The upper layer 17, which protects the sensing portion formed thereunder, is mainly formed of tantalum.
The operation of the above-described magnetoresistance device is as follows. If an external magnetic field is applied to the magnetoresistance device, a magnetized direction of the second magnetoresistance layer 13 and a magnetized direction of the second ferromagnetic layer 15 are varied. Thus, a magnetoresistance between the first ferromagnetic layer 13 and the second ferromagnetic layer 15 is varied. This variation in the magnetoresistance enables sensing of magnetic data stored on a magnetic recording medium, such as an HDD. In such a manner, data can be read from the magnetic recording medium using the variation in the magnetoresistance between the first and second ferromagnetic layers 13 and 15. When the magnetoresistance device is used, a magnetoresistive (MR) ratio of a varied magnetoresistance to a minimum magnetoresistance and an exchange coupling force Hex, i.e., a force required for a semi-ferromagnetic layer, to fix a magnetized direction of a second ferromagnetic layer should be maintained stable.
FIG. 1B illustrates a conventional TMR device 100. Referring to FIG. 1, the TMR device 100 comprises a lower layer 12, a first ferromagnetic layer 13, a tunneling barrier layer 18, a second ferromagnetic layer 15, a semi-ferromagnetic layer 16, and an upper layer 17, which are sequentially stacked on a substrate 11. In the TMR device 100, a tunneling current is varied according to a relative magnetization direction of a ferromagnetic material. Here, an MR ratio can be expressed as in the following Equation.                     MRratio        =                              highMR            -            lowMR                    lowMR                                    (        1        )            
A high MR ratio facilitates determining the spin directions of a fixed layer and a free layer. Thus, magnetoresistance devices that reliably record and reproduce data as “1s” and “0s” can be manufactured.
The manufacture of magnetoresistance devices with a high MR ratio requires formation of a barrier layer that has no pinholes, a low roughness of about 2 Å root-mean square or less, and a good insulating characteristic. To form a high-quality barrier layer, methods of setting optimal oxidation conditions or methods of forming a barrier layer using new materials are conventionally used.
The methods of setting optimal oxidation conditions are native oxidation, plasma oxidation, and ultraviolet oxidation. The native oxidation has advantages of a good surface morphology and a low resistance but lacks reproducibility and requires a prolonged process. Also, while the plasma oxidation enables a quick and simple process and reliable reproducibility, the resulting barrier layer has a non-uniform surface. Also, the ultraviolet oxidation allows formation of low-resistance devices but requires a prolonged process.
In the methods using new materials, barrier layers are formed of Ga2O3, AlN, AlON, TaOx, ZrOx, or MgOx. The barrier layers formed of these new materials have their advantages and disadvantages. That is, a Ga2O3 barrier layer, which has a low resistance, makes it impossible to measure its MR characteristic. An AlN barrier layer or an AlON barrier has a low bias voltage dependence but has a high resistance. A TaOx barrier layer facilitates coupling between a ferromagnetic layer and an insulating layer but has a low MR characteristic. A ZrOx barrier layer has an advantage of a low height as well as a disadvantage of a low MR characteristic. Also, while a MgOx layer has a low resistance and a small coupling between intermediate layers, it cannot be deposited beyond a limited height.
Meanwhile, sometimes a magnetoresistance device overheats during its manufacture or use. During use, a magnetoresistance device heats up to a temperature of about 150° C. due to an applied external current, and even to a higher temperature. During manufacture, the magnetoresistance device is heated to a temperature of about 300° C., which is higher than the temperature during use. If the magnetoresistance device is heated to a high temperature, atoms in each layer begin to move very actively, thus generating interdiffusion or intermixing of atoms between adjacent layers. The interdiffusion or intermixing of atoms is greatly affected by roughness of an interface between adjacent layers and the size of crystalline grains. More importantly, the principal characteristics of a magnetoresistance device, such as an MR ratio or an exchange coupling force, may be degraded due to the interdiffusion or intermixing.
In the above-described conventional magnetoresistance devices 10 and 100, if they are heated to a high temperature, the principal characteristics such as the MR ratio or the exchange coupling force are greatly reduced because of very active interdiffusion or intermixing. Also, incorrect sensing of magnetic information frequently occurs during use. In particular, a high-density magnetic recording medium, an applied magnetic field of which is small, has more serious problems.
Therefore, a magnetoresistance device requires a structure to prevent degradation of an MR ratio or an exchange coupling force even if a magnetoresistance thereof is heated to a high temperature. The above-described problems may occur in all kinds of magnetoresistance devices, such as magnetoresistance heads and MRAMs.